Asm variable in C code

Question

I am reading Secure Programming Cookbook for C and C++ from John Viega. There is a code snippet where I need some help to understand:

asm(".long 0xCEFAEDFE \n"
    "crc32_stored:    \n"
    ".long 0xFFFFFFFF \n"
    ".long 0xCEFAEDFE \n"
);

int main(){
    //crc32_stored used here as a variable
}

What do these lines exactly mean: "crc32_stored:\n", ".long 0xFFFFFFFF \n"? Is this a variable definition and initialization?

Trying to compile the code from the book I got the following error:

error: ‘crc32_stored’ undeclared (first use in this function)

Answer 1

crc32_stored: is simply a label, which in assembler is just an alias for a memory address. Since the label itself does not take up any space in the object code the address represented by crc32_stored is the address of .long 0xFFFFFFFF which assembles to four FF-bytes. In the object code the label will show up as a symbol, which means pretty much the same thing (just an alias for an address).

In C, a variable is (in a way) yet another way to express the same thing: A name that refers to a certain address in memory, but it has additional type information, i.e. int or long. You can create a variable in C with int crc32_stored = 0xFFFFFFFF; which (minus the type information) is equivalent to assembly crc32_stored: .long 0xFFFFFFFF, but that will create a different alias to yet another address.

You can tell the C compiler to not reserve a new address for the name "crc32_stored" but to create only the alias part and then to couple it with the address of a symbol with the same name. That is done with a declaration using the "extern" storage-class specifier, as in extern int crc32_stored. By this you "promise" to later link against another object file that will have this symbol.

Obviously you have to take care yourself that the C type information matches the intention of the assembly code (i.e. there are 4 bytes at the given address that should be interpreted as a signed 32-bit integer).

Addendum: Without the extra declaration the symbol is not visible from C code, because the assembler parts are processed separately. The symbols can not be exported to C code automatically because the type information is missing. (An assembly label does not even include information about whether it points to data or code.)